The use of reinforced plastics or composites in the fabrication of structural components has grown substantially in recent years. Composite structures are formed by overlapping layers of "tows", i.e., an essentially continuous band or strand of composite material consisting of a number of graphite, aramid, glass or other high strength fibers impregnated with a binder or matrix material either at the time of forming the composite structure or beforehand in which case the tow is also referred to as a "towpreg". Common matrix materials include tacky B-stage resins later cured to a thermoset stage, e.g., epoxies; non-tacky solid resins which melt and flow prior to curing to a thermoset polymer, e.g., bismaleimides; and, non-tacky thermoplastic resins processed entirely by melting and freezing, e.g., PEEK (polyetheretherketone) and PPS (polyphenylene sulfide). Composite structures have become increasingly popular as a replacement for metallic parts, particularly in high performance applications such as in the aircraft industry, because of the high strength-to-weight ratio, good corrosion resistance, good impact resistance and high electrical and thermal resistance exhibited by composite parts.
One aspect of the composites industry which has restricted the use o f composite parts in some applications is that traditionally many composite parts had to be fabricated by hand or with several manual operations. The technology of automating the formation of composite parts continues to evolve, but there are three currently available techniques which are effective to varying degrees in forming composite parts of different shapes from composite tows.
One automated technique of forming composite parts involves the use of filament winding machines which employ a wet winding procedure in which fibers of the filamentary material are drawn through a resin bath mounted on a traversing carriage having a pay out eye. A form or tool, carried on a rotating mounted structure, is located with respect to the carriage such that the resin impregnated fibers are guided under tension by the pay out eye longitudinally along the rotating tool. The pay out eye is effective to lay down the individual fibers side-by-side to form a band or tow in the form described above, and the pay out eye traverses the tool from end to end laying down successive layers of fiber tows until the desired wall thickness is built up on the tool. The resin or matrix material is cured on the tool, and then the tool is removed leaving the cured wound composite structure. See, for example, U.S. Pat. No. 3,363,849.
A second generation composite part forming device, commonly referred to as a tape laying machine, is disclosed, for example, in U.S. Pat. Nos. 3,616,078; 4,822,444; 4,273,601; 3,775,219; 4,292,108; and 4,419,170. Machines of this type lay down a relatively wide "tape", typically 0.007-0.014 inches (1.8-3.6 millimeters) in thickness and 3-12 inches (76-305 millimeters) wide, which comprise a pre-impregnated group of continuous tows or towpregs oriented parallel to one another on a carrier material such as paper and reeled for storage. The individual bands or tows are melded edge-to-edge on the carrier to form a substantially continuous composite tape of desired width. These tapes are carried in a placement head supported by structure capable of manipulating the placement head relative to a tool or form about a number of axes. Unlike filament winding devices, tape laying machines are capable of accommodating more complex shaped parts because the tows which form the tape are pressed or compacted directly onto the tool by a compaction roller or shoe carried on the placement head. The mechanisms which carry the placement head are effective to maintain the roller or shoe substantially perpendicular to the surface of the tool such that the tape is pressed against non-planar surfaces of the tool. As a result, tape laying machines are more versatile than filament winding apparatus for large gently contoured parts and have been effective in automating the production of some parts which had previously been constructed entirely by hand or with a number of hand lay up operations.
A third generation of automated devices for the fabrication of composite parts is disclosed, for example, in U.S. Pat. Nos. 4,699,683 to McCowin and 5,110,395 to Vaniglia, which is owned by the assignee of this invention. Apparatus of the type disclosed in these patents are referred to as "fiber placement" machines and differ from tape laying machines in that they apply a number of individual tows side-by-side onto a form or tool rather than a preformed tape that is reeled with a carrier material. Fiber placement machines include a creel assembly consisting of a number of reels or spools of tows or towpregs which are individually fed at independently controlled rates to a fiber placement head. The fiber placement head includes structure for handling each tow individually including guides or channels for transmitting each individual tow through the fiber placement head, cutters for individually cutting each tow, clamping mechanisms to clamp each cut tow and restart mechanisms to advance the cut tows to a compaction roller or shoe which presses such tows onto the surface of a tool. The fiber placement head feeds a predetermined number of tows, side-by-side, to the compaction shoe to form a relatively wide band consisting of a number of individual tows.
The effectiveness of each of the above-described methods and apparatus for forming composite structures depends, in different degrees, upon the dimensional accuracy with which the tows are formed. As mentioned above, in order to form a composite part using any of the above-described techniques, the tows or towpregs must be laid down side-by-side onto a tool or form, or onto a carrier material such as paper to form a tape which is then applied to the tool or form. If the width dimension of the tow or towpreg is inconsistent, i.e., has a side-to-side dimensional variation, the tows or towpregs can be laid down on the tool such that a gap is formed between adjacent tows, or such that adjacent tows overlap one another. Gaps between adjacent tows can produce weaknesses in the resulting composite parts, and undue overlapping of adjacent tows with one another can result in the waste of material and/or the production of composite parts having a greater wall thickness than desired.
The formation of composite tows having a uniform width dimension is particularly important in connection with the operation of fiber placement machines described in U.S. Pat. Nos. 4,699,683 and 5,110,395. As mentioned above, these types of devices employ a fiber placement head which includes a separate guide or channel for each individual tow through which such tows are advanced before they are pressed onto the surface of the tool by a compaction roller or shoe. If the tows are too wide, they can bunch up within the channels or guides and fail to pass through the fiber placement head. On the other hand, tows which are too narrow can be laid down by the fiber placement head such that gaps are formed therebetween on the form or tool.
The formation of individual tows, and composite "tapes" consisting of a number of tows side-by-side on a carrier material, has been addressed in the prior art. The formation of composite tapes is disclosed, for example, in U.S. Pat. Nos. 3,737,352 to Avis et al and 4,557,790 to Wisbey. In the Avis et al patent, a method and apparatus for forming a tape is disclosed which comprises directing a number of individual tows through a resin bath, drying them and then orienting the tows side-by-side within a groove of a shaft where they are melded together edge-to-edge by a roller inserted within the groove. Each of the individual tows are guided through the resin bath and into the shaft groove by different mechanisms such as a "comb", i.e., upright, side-by-side pegs, or, alternatively, a pair of stepped shafts each having alternating slots and raised lands wherein the slots of one shaft are axially offset from the slots of the second shaft so that the tows are supplied side-by-side to the tape forming groove. But no provision is made in the Avis et al apparatus to control the width dimension of each individual tow. As a result, the several tows are pressed together, side-by-side, between the forming rollers to form a composite tape in which spaces may be present between adjacent tows and/or wherein the adjacent tows may overlap one another.
In the Wisbey Pat. No. 4,557,790, an apparatus for making tape in situ on a tape laying machine is disclosed wherein a plurality of parallel, resin impregnated towpregs are deposited onto a paper carrier strip which is then run through compaction and back up rolls to flatten and widen the tows until contiguous tow edges are melded into a composite tape strip. As in the Avis et al disclosure, no provision is made in the Wisbey device for accurately controlling the width of each individual tow.
U.S. Pat. No. 4,877,471 to McCowin et al discloses a method and apparatus for delivering a resin impregnated multi-filament band. A plurality of rovings, each comprised of a bundle of resin impregnated filaments, are supplied from a creel assembly having multiple spools of rovings. Each roving is compacted to a predetermined thickness between a compaction roller and a back up roller, and then these compacted rovings are trained around guide rollers adjusted to present the rovings in side-by-side alignment to form a tow which is thereafter wound on a winding mandrel. An important limitation of the apparatus disclosed in the McCowin U.S. Pat. No. 4,877,471, and other systems of this general type, is that precise control of the roving width cannot be accomplished since the width is only indirectly controlled by flattening each tow to a uniform thickness. This is because no structure is provided to confine the side edges of the fiber bundle or rovings as they are compacted in the side-to-side or width direction. As a result, any variation in the cross-sectional shape and/or size of the rovings will produce a flattened tow having a uniform thickness but a non-uniform width. As mentioned above, this presents a particular problem in fiber placement machines where a relatively close tolerance must be maintained on the width dimension of the tow supplied thereto to avoid bunching up within the fiber placement head, or the formation of gaps between adjacent tows when they are compacted onto the form or tool.